Soot sensors, used to measure particulate in, e.g., engine exhaust, typically can be established by simple resistive devices. Such a sensor typically consists of a non-conductive substrate, most often alumina, with a screen print pattern using a conductive material, often a precious metal in order to withstand the temperature of a co-fire (although co-fire may not be necessary). The opposite side of the substrate can bear a heater to heat the soot sensor when needed to remove excess soot.
The soot sensing portion of the element consists of two electrodes with inter-digitized “fingers” that maximizes a perimeter between the two electrodes. When soot from the exhaust lands on the sensor, the carbon makes a high resistance short between the electrodes, effectively lowering the resistance. The more the soot collects, the lower the resistance, and this resistance is measured as an indication of the amount of soot. If it is desired to clean off the soot from the element, the heater on the element is activated to clean off the element.
While the electrodes of a soot sensor must be exposed to the exhaust stream in order to work, they must also be protected from being abraded away from the exhaust gas. U.S. Pat. No. 7,280,028 and application Ser. No. 11/750,883, owned by the present assignee and incorporated herein by reference, discuss such protection.
As understood herein, it is desirable to establish as long of a path between the two electrodes as possible, because once the carbon shorts the path, the more connections that are made, the higher the signal. Further, as also recognized herein the shorter the distance between the paths, the higher the sensitivity of the signal. With these recognitions in mind, the invention herein is provided.